Microwave antennas are widely used in communications, radio astronomy, radiotelemetry, radars, etc. It has also been widely proposed and experimented to use electromagnetic waves for energy transmission between two separated locations without use of physical connections. There is a need for a cost-effective means for the reception and conversion of electromagnetic power to direct current power more suitable for moving platforms on which the reception/conversion system is located. A rectifying antenna is customarily called a rectenna and includes antenna elements and rectifiers directly connected to them to produce a direct current output. An exemplary application of the rectenna in which this need arises is the provisioning of 30 KW or more of propulsive and communications payload power for lightweight electrically-powered aircraft. In operation, such aircraft would circle over fixed ground antenna systems, transmitting power in the 2.4 to 2.5 GHz microwave ISM band, for continuous periods of weeks or months at a time and relay communication signals between separated locations.
Of course, there are many other applications in which the supply of energy to a remotely located station is desired in the form of electromagnetic waves, thus eliminating the needs of physical connections, e.g. wires, pipes, and permitting the station to be movable. It is also advantageous to provide antennas which can perform equally well for microwaves of various polarizations.
Another application for antenna arrays at microwave and higher frequencies is their use in imaging arrays where information regarding the electromagnetic power incident on each discrete element of an array is desirable.
Copending U.S. patent application Ser. No. 07/124,159 filed on Nov. 23, 1987, now U.S. Pat. No. 4,943,811 (corresponding Canadian patent application No. 557,680 filed on Sep. 16, 1988) which has the present inventors as joint inventors, describes a dual polarization power reception and conversion system. This device consists of two orthogonal arrays of linearly-polarized thin film rectennas of specific format and element spacings. This antenna has proven to be highly efficient and to have a wide range of angles of reception. However, it has certain drawbacks in its manufacture, accessibility for testing and repair and power handling capability.
This prior-art system consists of two dielectric layers, each metal clad on both sides, in close proximity to each other (though electrically isolated). This requires a multilayer printed circuit board construction with its attendant difficulties and cost. In addition the thickness of the diode rectifiers used and the requirement for access to them involves the partial cutting away of one dielectric layer, a difficult procedure with thin flexible layers used.
Thirdly, the power handling capability of this prior art system is limited to one rectification unit for each polarization with power dissipation limited to radiative and convective cooling of the exposed foreplanes only. Power handling is also limited by the requirement of antenna spacings of at least half a wavelength (in free space). This element density limitation is due to the requirements for resonant half wave dipoles and isolation between the output filter of one element and the antenna of the adjacent unit.
Applicant's Canadian patent application No. 587,182 (filed on Dec. 28, 1988) describes an alternative power reception and conversion system. This design eliminates the manufacturing, installation and power handling problems discussed above but again is limited in its power handling by the requirement that the patch antennas be of resonant size (approximately half wavelength in the dielectric used). This criterion limits the number of elements that can be accommodated in any given area and hence the power handling capability of the array. There are a few references of interest. Thus "New Techniques for Combining Solid State Circuits" by Staiman in IEEE Journal Solid State Circuits, Vol. SC-3, Sept. 1968, pp 238-243, discusses separately spaced dipole antenna and "Field Theory of Guided Waves" by Collin, McGraw-Hill, New York, 1960, p 271, analyzes antenna reactances in small array antennas.